Filter for isotopic alteration of mercury vapor

ABSTRACT

A filter for enriching the  196  Hg content of mercury, including a reactor, a low pressure electric discharge lamp containing a fill of mercury and an inert gas. A filter is arranged concentrically around the lamp. The reactor is arranged around said filter, whereby radiation from said lamp passes through the filter and into said reactor. The lamp, the filter and the reactor are formed of quartz, and are transparent to ultraviolet light. The  196  Hg concentration in the mercury fill is less than that which is present in naturally occurring mercury, that is less than about 0.146 atomic weight percent. Hydrogen is also included in the fill and serves as a quenching gas in the filter, the hydrogen also serving to prevent disposition of a dark coating on the interior of the filter.

The Government has rights in this invention pursuant to subcontract4,524,210 under Prime Contract DE-AC03-76SF00098 awarded by the UnitedStates Department of Energy.

This is a continuation of co-pending application Ser. No. 947,216, nowU.S. Pat. No. 4,800,284, filed on Dec. 19, 1986.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to equipment for use in a photochemicalprocess, and more particularly to equipment for use in a photochemicalprocess for altering the isotopic composition of mercury.

BACKGROUND OF THE INVENTION

The excitation of specific mercury isotopes by photochemical means iswell known to the art. For example, the paper by Webster and Zare,"Photochemical Isotope Separation of ¹⁹⁶ Hg by Reaction with HydrogenHalides" J. Phys. Chem. 85, 1302 (1981) discloses such excitation.Mercury vapor lamps are commonly used ass an excitation source ofmercury isotopes for specific photochemical reactions. To be successful,photochemical separation of a single isotope requires that the spectralband width of the exciting mercury radiation must be sufficiently narrowto excite only the isotope of interest. The specificity depends upon thespectral band width of the source. The rate and extent of separation ofthe particular isotope from the feed stock can be strongly dependent onthe intensity of the radiation emitted from the mercury source.

A weakly ionized plasma of mercury and rare gases under low pressure, inthe order of 1 to 3 torr, forms the basis of the fluorescent lamp.Electrical energy is converted to natural mercury resonance radiation at253.7 nm. at an efficiency of 55 to 65%. This radiation, in fluorescentlamps, is converted to visible light by solid phosphors that are coatedupon the lamp envelope. The efficiency of the 253.7 nm. resonanceradiation emitted from exxcited mercury atoms in the plasma is absorbedand reemitted many times by ground state mercury atoms during its escapeto the walls of the discharge tube. This trapping of resonance radiationprolongs the effective lifetime of the excited atoms and increases theopportunity for radiationless energy conversion which reducesefficiency.

It is known that the 253.7 nm. resonance line of mercury is composed offive hyperfine components, principally the result of isotope shifting.As is known, the ¹⁹⁶ Hg isotope in natural mercury does not contributesubstantially to the radiation because of its low concentration, nordoes its emission and absorption heavily overlap with the otherhyperfine components. Therefore, by increasing its concentration, anadditional channel for the 253.7 nm. photons is provided which reducesthe average imprisonment time and increases radiation efficiency.

Devices have previously been disclosed to enrich the ¹⁹⁶ Hg in mercuryfeed stocks. In the paper of McDowell et al., "Photochemical Separationof Mercury Isotopes" CAN. J. Chem. Vol. 37, 1432 (1959), a disclosure ismade of reacting ²⁰² Hg(6³ P₁) atoms that are contained in naturalmercury with hydrogen chloride with a photochemical reaction in whichthe ²⁰² Hg atoms are excited during the reaction to precipitate a ²⁰²Hg₂ Cl₂.

As described in a paper delivered by Mark Grossman and Jakob Maya at theInternational Quantum Electronics Conference, June 1984, very highenrichment of ¹⁹⁶ Hg can be achieved in a photochemical reaction using anatural mercury vapor filter. When radiation from a microwave lampcontaining mercury enriched to 35% ¹⁹⁶ Hg is used in a filter, thefilter eliminates substantially all of the non-¹⁹⁶ Hg componentradiation permitting an isotopically selective primary excitation of the¹⁹⁶ Hg isotope. Selective excitation of ¹⁹⁶ Hg(6³ P₁) in natural mercuryvapor is obtained by an RF-excited, Hg and rare gas source whoseemission is filtered through an atomic vapor filter before it entersinto the reaction zone.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a filter forenriching the content of ¹⁹⁶ Hg in a mercury feedstock which flowsthrough a reactor, the filter comprising: an enclosure, at least aportion of which is transparent to the passage of ultraviolet light; afill of mercury vapor in said enclosure, said mercury vapor havingquantities of ¹⁹⁶ Hg less than that which is found in naturallyoccurring mercury.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view of an apparatus including a reactor vessel;and a lamp, utilizing the filter according to a preferred embodiment ofthe present invention.

FIGS. 2A and 2B are curves illustrating the relevant portions of thespectral energy distribution curves of, respectively, the emission of atypical low pressure lamp and the emission after the light has passedthrough the filter of the present invention.

FIGS. 3A, 3B and 3C are curves illustrating, respectively, the relevantportions of the spectral energy distribution curves of an evacuatedfilter, a filter with 10 torr of hydrogen, and a filter with 10 torr ofnitrogen.

FIGS. 4A, 4B and 4C are curves illustrating, respectively, the relevantportions of the spectral energy distribution curves of the emission froma filter operated at 24°, 16° and 8° C. The curves are based uponemissions from a low pressure microwave discharge mercury lamp with 2Torr argon and ¹⁹⁶ Hg in concentrations of 35 atomic weight percent. Thefilter contains mercury with 0.146 atomic weight percent and 10 Torrhydrogen.

For a better understanding of the present invention, together with otherand further objects, advantages an capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a novel composition of matterhoused in a UV-transparent filter for use in a system to enrich the ¹⁹⁶Hg content of mercury by a photochemical reaction process. The reactionis between "natural" mercury, or possibly previously ¹⁹⁶ Hg-enrichedmercury, and hydrogen chloride. The filter is particularly useful with asingle pass reaction vessel that surrounds the filter, which filter inturn, surrounds a low pressure electric discharge lamp having a fillcomprising mercury and an inert gas. The elements of the system, thelamp, the filter, and the reactor are formed of a material which istransparent to radiation at 253.7 nm, such as, for example, quartz orsilica. Examples of a low pressure (e.g., about 2.5 torr) electricdischarge lamp that are suitable for use in the system can be, forexample, any of the known low pressure discharge lamps which transmitradiation in the range of 253.7 nm., especially those using microwavecavities for the excitation of the mercury.

The filter of the present invention is disposed around the lamp and isformed of a pair of concentric cylindrical (or tubular) members that aresealed from the atmosphere. The concentric cylindrical members of thefilter are preferably sealed at their ends with a pair of spaced-apartend members, so as to form a cylinder with an axial passageway thatreceives the low pressure discharge lamp. The filter contains a fill ofmercury which is depleted in ¹⁹⁶ Hg so that the concentration of ¹⁹⁶ Hgis vbelow the quantity that is usually present in naturally occurringmercury, viz, below 0.146 atomic weight percent.

In another aspect of the invention, hydrogen is included in the filteras a quenching gas. The inclusion of the hydrogen has been found toinhibit the darkening of the interior of the filter. There may beseveral reasons for this. As an example, the hydrogen atoms formedduring the quenching of excited mercury atoms may react with mercuryoxide molecules that could form Hg and H₂ O. The result is a reductionof the oxide back to elemental Hg. Otherwise the mercury oxide wouldeventually form an opaque film on the inner filter wall.

The reactor is disposed about the filter and may take the form of a pairof concentric cylinders with inlet and outlet ports through which themercury feedstock, HCl gas, and other carrier gases can flow. Theexciting ¹⁹⁶ Hg radiation passes through the lamp envelope, through thefilter and into the reactor to produce an isotopic-specific reaction tothe ¹⁹⁶ Hg in the reaction vessel, whereby ¹⁹⁶ Hg₂ Cl₂ is formed.

Referring to FIG. 1 there is shown a low pressure mercury lamp 1comprising silica or a quartz discharge tube, equipped with microwavecavities disposed at one end thereof. For isotope separation of ¹⁹⁶ Hg,the inner diameter of the tube is preferably approximately 5 mm. Thedischarge lamp 1 typically contains an inert gas, e.g., argon, at apressure of approximately 2.5 torr and a mercury pressure ofapproximately 1 to 1.5 millitorr at about 20° C. Although argon ispreferred as an inert gas in the lamp, other gases such as neon may beused also.

The filter 3 includes a pair of concentric cylindrical members 3a and 3bspaced from each other at a distance about 1.0 centimeters. The filter 3is sealed from the atmosphere by a pair of spaced-apart end members 3cand 3d that are fused to the ends of the concentric cylindrical members3a and 3b. An axial passageway is formed in the filter 3 by the innercylindrical member 3b and is arranged to receive the lamp 1.

Reference is herein made to the co-pending application, filed on evendate herewith, Ser. No. 947,217, now U.S. Pat. No. 4,789,784 entitled"Apparatus For Isotopic Alteration Of Mercury Vapor" and assigned to thesame assignee as the present application. In a preferred embodimentdisclosed therein a means for controlling mercury pressure in the filtercomprises a tube 5, which is sealed at one end and in communication withthe interior of the filter 3 through port 5a, is disposed on the lowerend member 3d. A bead of mercury 6 is disposed at the closed end of thetube 5 and arranged so as to be in communication with the interior ofthe filter 3. Most preferably, a means for maintaining the mercury at apredetermined temperature is further described in the co-pendingapplication. The temperature maintaining means includes a sleeve 7disposed about the end of the tube 5 and around the bead of the mercury6. A sealing ring 9, such as a conventional O-ring, is disposed betweenthe sleeve 7 and the tube 5 to hold the sleeve 7 in place and preventleakage of heat exchange fluid (preferably water) which passes throughthe sleeve 7. The heat exchange fluid flows through a "T" connection 11,down sleeve 7 into heater 14 and thence to pump 12 to return to "T" 11.Pump 12 and heater 14 maintain the temperature of the tube at apredetermined level, so as to maintain a predetermined quantity ofmercury vapor in the filter.

The reactor 20 is disposed around the filter 3 and includes a pair ofspaced-apart concentric sleeves 20a and 20b. A conventional inlet portand outlet port 21a and 21b are disposed on the top and bottom of thereactor 20 to allow for the passage of the mercury feedstock.

As shown in FIG. 2A, the radiation from the low pressure mercury lamp 1,in the selected 253.7 nm area, has a principal emission peak and fivehyperfine peaks to the left of the principal peak. In FIG. 2B, theemission after the light has passed through the filter 3, the hyperfineemissions (peaks to the left of the ¹⁹⁶ Hg peak) have been suppressed,thereby reducing the interfering excitation peaks which enter thereactor 20, thereby in turn, reducing the chemical reactions between theHCl and non-¹⁹⁶ Hg isotopes.

The filter 3 is formed with at least a portion of its structure having aglass that is transparent to radiation at 253.7 nm, such as quartz, andis preferably entirely formed of quartz or silica, such that ultravioletlight can pass from the lamp 1 to the reactor vessel 3. The filter 3contains the previously described fill of depleted ¹⁹⁶ Hg-mercury, ascontrasted to "natural" mercury that contains 0.146 atomic weightpercent ¹⁹⁶ Hg isotope. When the ¹⁹⁶ Hg concentration is reduced belowthe levels present in natural mercury, the hyperfine emissions of amercury isotope other than ¹⁹⁶ Hg are reduced.

The absorption coefficient for mercury 196 component is given by theexpression: ##EQU1## where A=0.146×10⁻² for (¹⁹⁶ Hg) in natural Hg

T_(k) =vapor temperature in °K.

N_(tot) =total Hg density (cm⁻³)

In the model to be considered, radiation is assumed to pass normallyfrom the lamp through a one cm. filter.

At 50° C., N_(tot) =4.4×10¹⁴ cm⁻³ :

For natural Hg[¹⁹⁶ Hg]=0.146%, K₁₉₆ =0.376, and e^(-kx) =0.69;

For mercury rreduced in ¹⁹⁶ Hg to 0.063% (one-half the natural mercuryconcentration), [¹⁹⁶ Hg]=0.073%, K₁₉₆ =0.184, and e^(-kx) =0.83; and

For [¹⁹⁶ Hg]=0.0%, K₁₉₆ =0.0, and e^(-kx) =1.00.

The expression e^(-kx) represents the relative transmitted radiation orI/I_(o) as given by equation for Beer's law. This approximates theattenuation and together with the assumption of normal transmissionthrough the reactor gap, forms the "slab model".

                  TABLE I                                                         ______________________________________                                        Calculation Of Improvement In Transmission Of                                 .sup.196 Hg 253.7 nm Component Through A Filter                               Depleted In .sup.196 Hg Using A Slab Model At 50° C.                                                    %                                                         Absorption          Improvement                                  .sup.196 Hg  Coefficient                                                                             Exp       in Transmission                              Concentration                                                                              K.sub.196 (-K.sub.196 X)                                                                          (I)                                          ______________________________________                                        Natural Hg   0.376     0.690     0.0                                          50% Depleted .sup.196 Hg                                                                   0.184     0.830     20.0                                         100% Depleted .sup.196 Hg                                                                  0.00      1.000     45.0                                         ______________________________________                                    

As shown in the above Table and with a 1 cm. thick filter, a 20%improvement in transmission of narrow band radiation for ¹⁹⁶ Hgexcitation can be achieved for a 50% depletion of ¹⁹⁶ Hg in naturalmercury (0.073% ¹⁹⁶ Hg) and a 45% improvement in transmission can beachieved if all the ¹⁹⁶ Hg is removed.

The filter is filled with a mixture of quenching gas and mercury vaporand as previously mentioned, the filter contains the depleted ¹⁹⁶ Hg andhydrogen, the latter serving as a quenching gas. As mentioned above, theuse of hydrogen reduces the disposition of a dark coating on the insideof the filter, which has been observed to occur with other quenchinggases. Heretofore, nitrogen has been used to act as a quenching gas.When compared to using hydrogen as as quenching gas, a nitrogenquenching gas is less than fully satisfactory.

The mercury filter needs to absorb non-¹⁹⁶ Hg radiation which couldexcite non-¹⁹⁶ Hg isotopes, and the hydrogen serves as a quenching gas.Quite unexpectedly, it has been found that the hydrogen does not formstable by-products in the herein described mercury filter environment,which appears to be the case with other quenching gases previously used.When using hydrogen as a quenching gas, radiation of the mercury isconverted to non-radiative energy through collisions.

As can be seen in FIG. 3B, with a filter having 10 torr H₂, there is asignificant depression of the emission lines to the left of theprincipal ¹⁹⁶ Hg emission line in the curve. With a nitrogen fill, asshown in FIG. 3C, emission lines attributable to non-¹⁹⁶ Hg isotopesoccur. Such is also the case with the curve shown in FIG. 3A whichillustrates the lamp emission after passing through a filter containingonly mercury.

Without being limitative on the scope of the present application, thefollowing specific example is offered:

A filter having a thickness of 10 mm. with a fill of H₂ as a buffer gasat a pressure of 10 torr was used. The excitation lamp diameter was 5mm. and had a fill of argon and mercury and operated at a gas fillpressure of 2.5 torr at 20° C. The filter contained mercury that was 50%depleted in ¹⁹⁶ Hg (approximately 0.073 atomic weight percent ¹⁹⁶ Hg).The filter was operated at a temperature of approximately 40° C. whichprovides substantial non-¹⁹⁶ Hg isotope emission suppression. Thesuppression of the non-¹⁹⁶ Hg isotopes is shown in that in FIG. 4C at afilter temperature of 8° C., hyperfine emissions to the left of theprincipal ¹⁹⁶ Hg peak are less suppressed than at 16° C., as shown inFIG. 4B, and less suppressed than at 24° C. as shown in FIG. 4C.

The feedstock into the reactor was natural mercury which flowed at arate of 19 milligrams per hour, together with HCl at a rate of 100 sccm(standard cubic centimeters per minute) and argon at a rate of 80 sccm.Upon measurement, the mercury was enriched from the 0.146 atomic weightpercent ¹⁹⁶ Hg to 1.6 atomic weight percent. The enrichment of ¹⁹⁶ Hgcan be accomplished as disclosed in a co-pending application entitled:"High Feedstock Utilization Process for ¹⁹⁶ Hg Enrichment in aPhotochemical Flow Reactor", Ser. No. 947,218 now abandoned, filed oneven date herewith, and assigned to the same assignee as the presentapplication.

It is apparent that modifications and changes can be made within thespirit and scope of the present invention, but it is our intention,however, only to be limited by the scope of the appended claims.

What is claimed is:
 1. A filter for altering ¹⁹⁶ Hg isotope of a mercuryfeedstock including ¹⁹⁶ Hg isotope in a mercury feedstock which flowsthrough a reactor, said filter comprising:an enclosure, at least aportion of which is transparent to the passage of ultraviolet light; anda fill of vapor that produces radiation which excites a chemicalreaction in isotopic forms of mercury, and hydrogen included in saidenclosure.
 2. The filter according to claim 1 wherein the hydrogen is ata pressure between about 7 and 13 torr.